The present invention relates to the electrical drive for reflective memory displays.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
Current flat panel displays use two transparent glass plates as substrates. In a typical embodiment, such as one set forth in U.S. Pat. No. 5,503,952, a set of electrical traces is sputtered in pattern of parallel lines that form a first set of conductive traces. A second substrate is similarly coated with a set of traces having a transparent conductive coating. Coatings are applied and the surfaces rubbed to orient liquid crystals. The two substrates are spaced apart and the space between the two substrates is filled with a liquid crystal material. Pairs of conductors from either set are selected and energized to alter the optical transmission properties of the liquid crystal material. Such displays are expensive, and currently are limited to applications having long lifetimes.
Fabrication of flexible, electronically written display sheets using liquid crystals materials is disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. The structure has the capacity of maintaining one of the given states in the absence of an electric field.
In xe2x80x9cLiquid Crystal Dispersionsxe2x80x9d, World Science, Singapore, 1995, page 408, Paul Drzaic discusses the electrical drive of cholesteric liquid crystal displays. Drzaic also states on page 29 that xe2x80x9cThe use of gelatin, however, creates a material that is too conductive for practical use in electrically addressed PDLC systems.xe2x80x9d. Drzaic further states xe2x80x9c . . . actual displays require AC signals to prevent electrochemical degradation.xe2x80x9d Subsequent patents follow Drzaic""s assumptions. Later patents such as U.S. Pat. Nos. 5,251,048 and 5,644,330 and 5,748,277 all require AC fields having a net zero unipolar field for matrix cholesteric liquid crystal displays to prevent ionic destruction of the display. The cited patents have display structures formed using expensive display structures and processes applicable to long-life situations which require AC drive schemes.
The drive schemes require that each element be written using alternating electrical fields that provide a net zero field across the display to prevent ionic migration. AC drives require large numbers of power supplies and large numbers of switching elements per line.
The development of simple processes to manufacture inexpensive flexible displays will generate new markets of inexpensive, short lived displays that will benefit from a lower cost electrical drive circuitry.
It is an object of the present invention to provide low cost memory displays generated using coated polymeric dispersed cholesteric liquid crystals on flexible substrates.
It is another object of the present invention to provide a simpler, lower cost method of driving coated polymer dispersed cholesteric materials on flexible substrates.
These objects are achieved by a reflective memory display, comprising:
a) a flexible polymeric substrate;
b) a patterned first conductor disposed over the substrate;
c) a layer including a polymeric dispersed cholesteric liquid crystal material coated over the first patterned conductor;
d) a patterned second conductor disposed over the polymeric dispersed cholesteric liquid crystal; and
e) control means for repeatedly applying selective unipolar voltages across particular portions of the patterned first and second conductors to cause electric fields which are applied to portions of the polymeric dispersed cholesteric liquid crystal layer to change its reflectance thereby presenting a series of different reflectance images.
The invention reduces the number of voltages required to drive such a display as well as reducing the number of voltage switching elements.
It is a feature of the present invention that it eliminates the need to work with glass substrates with associated complex manufacturing techniques. The present invention by making use of flexible polymeric substrates permits the use of unipolar fields to change reflectance. A unipolar drive reduces the associated cost of the drive circuit.